Re: [PATCH v3 5/6] kasan: Unset panic_on_warn before calling panic()

Dmitry Vyukov
2020-01-16T05:23:19Z

On Thu, Jan 16, 2020 at 2:24 AM Kees Cook <keescook@chromium.org> wrote:
>
> As done in the full WARN() handler, panic_on_warn needs to be cleared
> before calling panic() to avoid recursive panics.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> mm/kasan/report.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 621782100eaa..844554e78893 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -92,8 +92,16 @@ static void end_report(unsigned long *flags)
> pr_err("==================================================================\n");
> add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> spin_unlock_irqrestore(&report_lock, *flags);
> - if (panic_on_warn)
> + if (panic_on_warn) {
> + /*
> + * This thread may hit another WARN() in the panic path.
> + * Resetting this prevents additional WARN() from panicking the
> + * system on this thread. Other threads are blocked by the
> + * panic_mutex in panic().

I don't understand part about other threads.
Other threads are not necessary inside of panic(). And in fact since
we reset panic_on_warn, they will not get there even if they should.
If I am reading this correctly, once one thread prints a warning and
is going to panic, other threads may now print infinite amounts of
warning and proceed past them freely. Why is this the behavior we
want?

> + */
> + panic_on_warn = 0;
> panic("panic_on_warn set ...\n");
> + }
> kasan_enable_current();
> }